The present invention relates to methods of preparing electrodes for batteries and other electrochemical devices. More specifically, the invention relates to a freeze-drying technique to minimize redistribution of electrode components during solvent removal.
Electrodes are critical components of batteries, fuel cells, supercapacitors, and other electrochemical devices. Typically they are prepared by mixing various components necessary for proper operation of the electrode in a liquid carrier such as a solvent. The components are well mixed with one another in the carrier to form a slurry. The slurry is then coated onto a current collector or other support and allowed to dry.
During the drying process, the liquid carrier is driven off by evaporation. This typically precipitates dissolved species and redistributes both soluble and insoluble electrode components so that some segregation occurs. Thus, while the components may have been initially well mixed in the slurry, they segregate and agglomerate to some extent during evaporation. This segregation may be due to capillary forces, natural affinities of the materials, variations in solubility, different settling rates of the components, and other physical and chemical factors.
Generally, the electrode components should be very well mixed to achieve high utilization. In a battery electrode, for example, an electroactive material must be immediately accessible to both electrons and ions in order to provide electrochemical energy. In many cases, the electrochemically active material does not conduct ions or electrons or both. In such cases, the electrode must include an ionic conductor and an electronic conductor in addition to the electroactive material. To obtain electrochemical energy from a given molecule of the electroactive species, it must be intimately contacted by both the ionic conductor and the electronic conductor. Obviously, the segregation of these components during the drying step of a conventional electrode fabrication process reduces the available capacity of the electroactive species.
A related problem occurs when an electronic or ionic conductor species becomes segregated to the point where they fail to form an interconnected matrix allowing a continuous path from an ion or electron source to the electrode interior. The electron source is usually a current collector which may be a metal plate mounted on the back of the electrode. The ion source is usually the electrolyte which contacts the opposite side of the electrode. If the electrolyte is a liquid, it may permeate through a porous electrode and itself serve as the ionic conductor. When an electronic conductor or an ionic conductor, if needed, forms disconnected islands which do not have a clear path back to the source of electrons or ions, the electroactive material contacting such islands may not be available to generate electrochemical energy.
It is also very difficult to produce relatively thick electrodes (e.g., of thickness greater than about 300 micrometers) from liquid carrier processes. This is because a "skin" will form on the electrode surface during the evaporation. This skin reduces the rate at which evaporation can take place. In essence, the skin forms a barrier to the transport of vaporized carrier out of the electrode. This may greatly increase the length of time required to evaporate the carrier and thereby form the electrode.
What is needed, therefore, is an improved process for preparing multi-component electrodes.